The present invention relates to a tire air pressure monitoring device and a pneumatic tire having a tire air pressure monitoring device. More specifically, the present invention relates to a tire air pressure monitoring device which can save power consumption and a pneumatic tire having the tire air pressure monitoring device attached thereon.
Tire air pressure monitoring devices are generally configured to detect physical quantities such as air pressure or temperature inside a tire cavity filled with air by a sensor and to send the detected signals to a vehicle side by a transmitter. Tire air pressure monitoring devices of this kind are attached inside a tire cavity. Thus, a battery is usually used as a power source for the monitoring device.
However, the above tire air pressure monitoring devices constantly transmit detected information to a vehicle side by a transmitter, and the maximum electric power is consumed during the transmission. Accordingly, a battery is drained quickly and required to be replaced frequently.
To counteract this problem, a tire air pressure monitoring device has been proposed, in which a transmitter is switched between a transmission mode and a stop mode by a centrifugal switch. As a vehicle starts to drive, tires rotate, centrifugal force acts on the centrifugal switch, and the centrifugal switch is turned on. Accordingly, the transmitter is switched to the transmission mode. When the vehicle is not driving, centrifugal force does not act on the centrifugal switch. Thus, the centrifugal switch is off, and the transmitter is switched to the stop mode. Therefore, power consumption is saved because of turning the transmitter to the transmission mode only when the vehicle is driving.
However, the centrifugal switch does not work unless a predetermined degree of centrifugal force or more acts on the switch. In other words, the centrifugal switch does not work until the traveling speed of the vehicle reaches approximately 8 km/h or more. Thus, tire air pressure cannot be monitored from starting of driving a vehicle to the traveling speed thereof which reaches that speed. To secure traveling safety of a vehicle, it is preferable to notify a driver of the tire air pressure information immediately after a vehicle starts to drive, at the latest. However, there is a disadvantage that the notification can not be done.
As another solution, there has been proposed a tire air pressure monitoring device which saves power consumption by using a vibration sensor to switch a transmitter between a transmission mode and a stop mode. Since this device determines whether the vehicle is driving by vibration of a vibration piece inside the vibration sensor, the device is able to notify a driver of the tire air pressure information immediately after the vehicle starts to drive. However, when a vehicle reaches a certain degree of traveling speed, a large centrifugal force acts on the vibration sensor. Consequently, the centrifugal force pressures the vibration piece in a direction where the centrifugal force is acting, and the vibration piece becomes incapable of vibrating. As a result, a driver cannot be notified of the tire air pressure information.